This is what Jonathan Katz does: Speak softly and carry a small shtick. It's an act familiar not just to the cult of viewers who still speak reverently of his animated series for Comedy Central, ''Dr. Katz: Professional Therapist," but also to the comedy club audiences and late-night TV watchers who have seen him do his wry, dry, hands-in-pockets stand-up over the years.

It's an act he'll be doing this weekend in a rare local appearance at Jimmy Tingle's Off Broadway in Somerville. But it's also not exactly an act.

''When he's doing stand-up or a talk show," says his wife of more than two decades, Suzan Kaitz, ''that's very much who he is."

For someone who is having a conversation with Katz, there will be a lot of laughter, plenty of deadpan commentary on the petty amusements and annoyances of daily life, but perhaps not so much deep personal revelation. ''I have a way of deflecting questions," Katz says at one point -- a sentence that, as understated one-liners go, ranks among his finest.

But over the past year or so, Katz, who is 58, has been making an effort to answer more questions -- and maybe even to pose a few of his own -- about something he has been dealing with since 1997 but only made public more recently: a diagnosis of multiple sclerosis.

''What I'm trying to do in my life," he says, ''is figure out what it means to be a comedian with MS, as opposed to a carpenter with MS or a secretary with MS: saying things that are slightly inappropriate and getting paid for it."

Sometimes that means retooling earlier jokes for this situation: ''My first neurologist was very holistic. He said no alcohol, no red meat, no salt. I said, 'What about sex?' He said, 'I'm seeing someone.' " Sometimes it means coming up with new material, like his I'm-so-competitive line about a fellow celebrity with MS: ''I'm gonna make Teri Garr wish she had lupus."

Sometimes it means using humor to deal with the realities of living with a progressive neurological disease, like fooling around with the scooter he uses to save his energy, marking its speedometer with a tortoise and a hare -- and saying he's ''not afraid to run it up to full bunny." And sometimes it just means doing jokes that have nothing at all to do with the damn disease, like his bits with his ''Dr. Katz" collaborator, animator Tom Snyder, about a venison-only restaurant, or ''The Re-Enactors' Studio," or a guy who's on Weight Watchers and death row at the same time.

All of that is likely to be on the agenda in his act tonight and tomorrow. He's planning to do a kind of live radio show, along with playing clips from his animated shows and from a full-length animated documentary he's been working on with Snyder, ''The Traveling Talent Show." Snyder will appear with him, as will bassist Andrew Blickenderfer and comic Bill Braudis, who was Dr. Katz's first patient.

''This is a very big deal for me," Katz says. ''It's different from anything I've ever done and also the same as everything I've ever done."

At its core, of course, ''it's going to be stand-up comedy," he says. ''But I'm not planning to stand up."

Breaking the rules

''When you hear a diagnosis as major as MS, it's something that terrifies. It's the kind of disease where you recognize things in retrospect," Katz is saying, and you could be forgiven for thinking that he's about to launch into the kind of solemn, had-I-but-known monologue of impending doom that typifies network prime-time narratives of illness. Not to worry: This is what's known to seasoned comedic professionals as a setup.

Sitting in his office, on the lower level of his Newton home, Katz flashes an X-ray on his computer screen; it's part of the Power Point presentation he has used in a routine for groups of MS patients. He explains, his deep-set dark eyes still solemn, that he and his brother-in-law had for years run against each other in a road race. ''In 1995, my brother-in-law won the race for the first time," Katz intones, then explains that he lost only because he had fallen down. He points to the screen. ''The X-ray shows I was ahead at the time I fell."

Katz says he has enjoyed doing this presentation for people with MS. (Other highlights include a list of diseases that must be ruled out before making a diagnosis, such as ''Lyme Disease, Puppy Love, Lemon-Lyme Disease, Lou Gehrig's Disease, Lou Gehrig's Sister's Disease.") But it's tricky sometimes to make just the right joke; he used to kid about wanting to make the Appalachian Trail wheelchair-accessible, until someone told him that that project was in the works.

What's even trickier, though, is to get him to talk about the disease and its effectson him in any detail. If pressed, he will tell you that he takes a medication for it, the maker of which used to sponsor his MS-group gigs; he will tell you that he can still walk but uses the scooter to reserve his energy for things that matter more to him than walking, that he gets very tired, that his balance is off, that he often feels stiff. But ask even a slightly probing question, such as what form of MS he has, and it's back to the jokes. ''I have extra-Jewish MS," he says.

''He will not complain," says Snyder, his longtime friend and collaborator. ''He can tell what his fears are, but he just will not complain. I think he has a sense of what one owes to the people around them, which is cheerfulness."

Katz disagrees, up to a point. ''There is a little whining involved, and complaining," he says. ''You know, it's not fun." He corrects himself. ''The scooter is fun."

In short, as he often puts it, ''I have two incurable diseases. I'm a comedian, and I have MS. And neither of them is getting a lot better."

Snyder, too, describes Katz's comedy as ''incurable," noting that it crops up even in the grimmest moments. ''If something awful happens on the news, it's always the comic side of the dark side," Snyder says. ''He even called me the morning my mother died, and he said, 'It is so hard to know what to say at a time like this. Especially with the three-hour time difference.' "

Suzan Kaitz tells that story, too, as evidence for her characterization of her husband as ''an unusual guy."

''You know how great that is?" she says. ''I love that he breaks those rules, because they're not necessary. People will often look at me and say, 'Is he kidding?' "

Doing it for fun

Picture, for example, the Hollywood receptionist who encountered Katz when he and Snyder were pitching a show a couple of years ago. In what Kaitz calls a gesture ''that just characterizes friendship in such a profound way," Snyder had procured a scooter to match Katz's.

''So we would arrive everywhere on two scooters," Snyder says. ''When we arrived at the Fox building, we were both sitting below the receptionist's desk. He said, 'Yes, we're here for the ''Handicops" interview. It's a cop show. They're both handicapped.' "

Well, no. They were really there to pitch ''Say Uncle," an animated comedy about family life that would have featured not just Katz's voice but Lisa Kudrow's. They got Fox to agree to a pilot, but the network later declined to pick up the series.

Katz is philosophical about the rejection. He's got other projects; there's the movie with Snyder, for one, and the stand-up, and the book he's working on, and the regular call-in comedy gig on ''The Next Big Thing" for Public Radio International.

''If Fox had picked up my TV show, it would have a budget of $1.2 million per episode," he says. '''The Next Big Thing' is $800 an episode. When I asked them for another $200, they said no."

Obviously, it's not about the money. And it's not really about hard work, either -- at least not according to Snyder.

''It's just constantly trying to feed our habit," he says of their projects together. And, he says, he needs Katz to remind him of that. ''I'll get kind of overwhelmed with devilish details, and he'll say, 'You have to remember that this is for fun. We're doing this for fun.' "

Katz can afford to be sanguine, given that he has ''Dr. Katz" on his resume (along with scriptwriting credits, with his friend David Mamet, for ''House of Games" and other films). ''Dr. Katz" won a slew of awards, including an Emmy for Katz's voicing of the neurotic analyst, and was beloved during its 1995-2000 run for its guest stars -- just about everyone in comedy ''except Bob Newhart," Snyder says, still pained by the irony of failing to snag TV comedy's other famous shrink -- and for Katz's aforementioned dry, wry brand of humor.

That is a brand, admittedly, that doesn't sell everywhere. Katz looks back with amusement on his first big break, two appearances on David Letterman's show in 1985.

''It made me a national act, which meant I could go to Dayton and get paid," Katz says. ''But people in the '80s wanted to be titillated and insulted. And I was doing stuff like: 'My aunt died last week. She was cremated. [Pause.] We think that's what did it.' "

If that's the kind of line that puts the ''dead" in ''deadpan," there's one person who can always be counted on for a laugh: his wife.

''I'm always willing to laugh when no one else is; I'll be the only laugher you hear in the room," Kaitz says. But that doesn't mean she laughs at everything. ''We have an agreement. What strikes one person as funny won't strike another. So I don't say, 'I don't think that's funny.' I say, 'It doesn't work for me.' That's been very helpful over the years."

What's also helpful, she says, especially in the face of a sometimes frightening and always unpredictable disease, is that they still do make each other laugh. ''I remember when our children were little," Kaitz says of their two daughters, now 22 and 13. ''We were never very capable of getting children to sleep through the night; it was not one of our talents. But after getting back into bed for the fifth time, we would be giggling so hard that we would wake them up the sixth time."

Just the thought of it makes her laugh. ''We have our somber and serious moments," she says. ''But then we have that."

Katz has his own theories on what makes their relationship tick. ''You have to keep a marriage fresh," he says. ''This is our trick. Every morning, for 24 years, my wife asks me, 'How do you take your coffee?' It's a small thing, but it's annoying."

Not just two lettersMaybe not then, maybe not often, but in his actions and offhand remarks, you do get a sense of what really matters to Katz. He and Kaitz moved to Newton from his native New York City because they considered it a better place to raise children; he is protective of his daughters and concerned for their privacy, asking that they not be named in print. And he speaks with some pain, however masked, of the family's move a couple of years ago from their home of 17 years, a Victorian in Newton Centre, to the modern ''Florida style" house that was more practical for someone with limited mobility.

''We moved here for the lack of stairs," Katz says. ''We gave up a lot." He takes off his glasses -- ''because I don't want to see all the details," he says. He's quiet for a minute. ''There is a part of me that is very maudlin about it. It's not something I had in mind."

He starts another anecdote, about seeing a neurologist for the first time after his diagnosis. ''It still hadn't hit me that this is a disease, not just two letters. And afterward I remember thinking, 'When I wake up, will I be able to walk tomorrow? Will I wake up without sight one day?'

''I try to enjoy the ride. A lot of it's not enjoyable. You don't become better at things because of MS."

At first, he confesses, he avoided going to meetings of people with MS. ''You don't want to see people in worse shape than you," he says. ''Part of me doesn't want to think that's what I have. But I have a good friend who's had MS most of her life. I asked her, 'Do you get these funny tingling sensations in your feet?' She said, 'Jonathan, I haven't felt my feet in more than 10 years.'

Katz pauses. ''It's all relative," he says. He pauses again. If there's a punchline, it doesn't come.